This invention relates to a rechargeable electric battery having a plate block arranged in a plastic block box and comprises positive and negative electrodes, which are electrically isolated by separators and are conductively connected by a sulfuric acid electrolyte, the box being closed off by a plastic cover which has closure plugs fitted in a gas-tight manner and/or acid level or acid density indicators fitted in a gas-tight manner. This invention also relates to closure plugs and acid level or acid density indicators suitable for such rechargeable batteries.
A mixture of hydrogen and oxygen, which react very easily, is produced during the charging of lead-acid batteries. Although ignition of this charging gas mixture very seldom occurs, as measured by the widespread and increasing number of these electrical storage devices, which are located almost exclusively in motor vehicle applications, methods of avoiding such ignition are continually sought. As a result of design improvements in such rechargeable batteries, the most frequent causes of charging gas ignition with consequential injury, such as recharging with devices external to the vehicle (31%), clamping or unclamping the battery cables (26%), starting-aid operations (19%) and refilling with distilled water, still only hide a very low risk of explosion.
The common factor in the above-mentioned ignition situations is in mishandling of the battery beforehand, which leads to formation of a spark with a sufficient energy content ( greater than 0.2 mJ), which then ignites the charging gas leaving the rechargeable battery and flashes back into the battery. The openings which come into question for the flashback are primarily the battery cell plugs provided with a small hole for the purpose of degassing and, in the case of specific designs with leakproof plugs, the common gas outlet from a plurality of cells, connected by a collecting line in the battery cover. Progress could be achieved by introducing antimony-free lead alloys for the current-carrying structural materials in batteries for the application in passenger cars, and the charging gas development, which has become considerably lower as a result, in conjunction with the use of battery covers with a gas collecting channel in front of whose opening, on the outside, there is a microporous molding (frit) which prevents a charging gas detonation flashing back into the battery. In parallel with this development, in rechargeable batteries of the above design, either gas-tight cell plugs are used or plugs and the associated opportunity for refilling with water are dispensed with entirely, if the antimony-free lead alloys already mentioned are use.
Thus, the risk of charging gas ignition from the interior of the rechargeable battery substantially still only comprises the following; in general brought about by the fracture of current-carrying structural materials which are not completely covered with battery acid. But here, too, in particular with regard to corrosion resistance and increase in mechanical strength, considerable success has been achieved. This is reinforced still further by the lower charging gas development and the resulting on average considerably higher level of acid.
Nevertheless, both in the case of passenger car batteries and, in particular in the case of utility vehicle batteries, in the manufacture of which antimony-alloyed lead is still used to a very great extent, charging gas ignitions still occur. Static electricity is regarded as the main cause in these cases which, to a very great extent, is produced by friction on the surface of the plastic battery housing.
On its route to the electrolyte or to the conductive structural material of the rechargeable battery, the charge of static electricity jumps over regions in the interior of the battery and, as a result of spark formation, leads to charging gas ignition. The latter is present more frequently in an ignitable concentration, in particular, in the case of the above-mentioned safety constructions having a frit element closure which, however, prevents the outflow of charging gas.
It is known to reduce the surface resistance of the block box cover by means of electrically conductive coatings or labels or by means of electrically conductive surface engraving (EP 841146 A1, EP 887870 A2) or to provide a Faraday cage by means of an electrically conductive covering of the battery with grounding to the vehicle ground (battery negative terminal) to avoid the build-up of static surface electricity.
It is also known to effect considerable attenuation of possible explosions, as far as their complete suppression by blocking up the cavities filled with gas. (EP 574619 A1, DE 42 32 961 A1).
However, it has been shown that even by means of these known measures, ignition of the charging gases cannot always be avoided. It would, therefore, be highly advantageous to provide a rechargeable battery of the type mentioned above in which ignition of charging gas in the interior of the battery caused by static electricity are avoided to the greatest possible extent.
The invention relates to a rechargeable electric battery including a plate block arranged in a plastic block box, positive and negative electrodes located in the box and electrically isolated by separators and conductively connected by sulfuric acid electrolyte, a cover for the box which has closure plugs and/or acid state indicators fitted in a gas-tight manner to openings therein, wherein at least a portion of an inner surface of the battery is electrically conductive or is provided with an electrically conductive layer, beginning in an area of a sealing seat of the closure plug or of the acid state indicator, and is electrically conductively connected to the electrolyte.
The invention also relates to a closure plug for a rechargeable battery having a plate block which is arranged in a plastic block box and comprises positive and negative electrodes isolated by separators and are conductively connected by a sulfuric acid electrolyte, including a closure plug body which is electrically conductive, at least at surface portions thereof, or is provided with an electrically conductive layer.
The invention further relates to an acid state indicator for a rechargeable battery having a plate block which is arranged in a plastic block box and comprises positive and negative electrodes isolated by separators and are conductively connected by a sulfuric acid electrolyte, including an acid state indicator which is electrically conductive, at least at surface portions thereof, or is provided with an electrically conductive layer.